©
2017 Sensata Technologies
Page 24
Installation
2.6.2
Select the Correct Conductor
To properly determine the required ampacity of the conductors in the PV System circuits, the NEC
requires a comparison of conductors that are sized with different adjustment factors. A conductor
sized to continuously carry the maximum current is calculated and compared against a conductor
sized with conditions-of-use factors applied. The conductor size with the greatest ampacity—after
it is associated with the lowest temperature rating of any terminal in the circuit—between the two
is selected.
•
Continuous Current Calculation
- The first calculation takes into consideration that the
maximum current in PV circuits is considered to be continuous, which is de
fi
ned as circuits
being able to deliver current for more than three hours. Therefore, the maximum continuous
current of these circuits is multiplied by 125% to ensure the conductors do not carry more
than 80% of their rated value (I
CONT
= I
MAX
x 1.25).
Info:
For PV source circuits and the PV output circuit, the
fi
rst required 1.25 multiplier
is to account for sustained periods when the sun’s intensity (irradiance) can cause
module current outputs to be greater than their rated STC irradiance value. The
second 1.25 multiplier is related to the NEC requirement to keep conductors from
normally operating above 80 percent (1 ÷ 0.80 = 1.25). These two multipliers taken
together is referred to as the 156% rule, because 1.25 × 1.25 = 1.56.
•
Conditions-of-Use Calculation
- The second calculation adjusts for worst-case installation
conditions. It requires adjustment and correction factors (commonly referred to as conditions-
of-use) to be applied to the maximum circuit currents. Conditions-of-use factors take into
account the number of current carrying conductors that are close to each other when run in
conduit (i.e., conduit
fi
ll), and the temperature to which the conductors are exposed.
Info:
The condition-of-use calculation is required because large adjustments due
to multiple current-carrying conductors and high-temperature operation can result
in reduced ampacity, which often results in a conductor that is larger than the
conductor sized for continuous current.
o
Conduit Fill Correction - Conduit
fi
ll refers to the number of current-carrying wires of a
particular size and type allowed in a particular size of conduit. Adjustments to the conductor
ampacity is required if there are more than three current carrying conductors in conduit
or bundled together (excludes grounding conductors) in continuous lengths greater than
24 inches. Use the conduit fill adjustment factor in Table 2-5 after determining how many
current carrying conductors in a conduit/raceway run greater than 24 inches.
o
Temperature Correction - As is common in PV systems, conductors can operate at extreme
temperatures, which can reduce the conductor’s overall ampacity. If the wire is subjected
to ambient temperature conditions greater than 30°C or exposed to sunlight on or above
rooftops, the size of the conductors will need to increase to compensate for lost ampacity.
For temperature correction calculations,
fi
rst determine the local ambient temperature
1
. If
the average high temperature is greater than 30°C, then an ambient temperature factor is
required to be applied to the ambient temperature (see Table 2-3). Also, if the conductors or
raceways are exposed to sunlight on a roof, code requires an additional rooftop temperature
value (or adder) that must be added to the average high ambient temperature based on
the height of the conductor/raceway from the roof surface (see Table 2-4).
Note:
If you use the Solar ABC’s website for high temperature data, the rooftop temperature
adder is already factored in the
Distance above roof
calculations.
Note 1:
Many PV designers use the 2% average high temperature data from the ASHRAE Handbook -
Fundamentals to find local ambient temperatures. This data is readily available using the interactive map at
the Solar ABCs website
(http://solarabcs.org/about/publications/reports/expedited-permit/map/index.html)
.
While the high temperature data is recommended for conductor ampacity sizing, more conservative cold
temperature data is recommended when sizing for array voltage (which can damage the PT-100 controller).
See the caution note in Section 2.5.2 for more information.